Use of organic materials having high nonionic charge carrier mobility

The present invention relates to the use of organic materials having low electroconductivity but high nonionic charge carrier mobility, as charge transport media.

EP-A-0 527 376, of which U.S. 5,393,626 is an English language counterpart, describes low-molecular-weight and polymeric organic photoconductors having generally discotic liquid-crystalline properties and increased photoconductivity. A typical representative is hexaalkoxytriphenylene, which has a charge carrier mobility of almost 10 -3 cm 2 /Vs (cf. D. Adam, F. Closs, T. Frey, D. Funhoff, D. Haarer, H. Ringsdorf, R. Schuhmacher, K. Siemensmeyer, Phys. Rev. Lett., 70 (1993) 457).

The article "Functionalized Porphyrin Discotic Liquid Crystals: Photoinduced Charge separation and Trapping" by Marye Anne Fox, Allen J. Bard, Horng-Long Pan and Chong-Yang Liu in Journal of the Chinese Chemical Society, 40 (1993) 321-327, states that individual members of a family of highly absorbent porphyrins which contain symmetrically arranged flexible side groups form discotic liquid-crystalline mesophases at moderate temperatures. The arrangement of these phases is maintained when a layer of this material is cooled to room temperature. If such films are exposed to visible radiation in the presence or absence of an electric field, photovoltaic effects occur.

The article "Synthetic Design of Liquid Crystals for Directional Electron Transport" by Marye Anne Fox and Horng-Long Pan in Photochemical processes in Organized Molecular Systems, 1991, 359-376, describes the production of a solid-state photovoltaic cell comprising a thin film of an organic semiconductor arranged between indium tin oxide electrodes.

In the above cases, an increase in the charge carrier concentration achieved by light absorption is necessary in order to achieve adequate conductivity.

WO-A-94/05045 describes heterocontacts comprising conjugated polymers and acceptors, diodes, photodiodes and photovoltaic cells, where, for example, fullerenes, in particular buckminster-fullerene, C 60 , are employed. German Patent Application P 43 39 711.5 describes novel triphenylene compounds and processes for the preparation of crosslinked, discotic, liquid-crystalline polymers.

As charge transport medium in electrochemical cells, the charge transport has hitherto been effected by mobile ions which must have an adequate diffusion rate. In electronics components, the charge transport media employed hitherto have been inorganic semiconductors or organic polymers or oligomers with a sufficiently high charge carrier concentration due to doping or photoexcitation. The only components having a charge transport medium of low conductivity which are interesting for practical applications are photovoltaic cells, in which intrinsic inorganic or organic semiconductors are rendered conductive through photoexcitation, or in which an inorganic high-resistance semiconductor is used for charge transport after dye sensitization. High-resistance organic materials which effect charge transport on their own have not been disclosed hitherto.

It is an object of the present invention to provide organic materials which do not have the disadvantages outlined above.

We have found that this object is achieved by the use of organic materials having a specific conductivity of less than 10 -2 S/cm and a nonionic charge carrier mobility of greater than 10 -4 cm 2 /Vs as charge transport medium, with the proviso that an increase in the charge carrier concentration by a factor of 10 or more is not caused in this organic material by light absorption.

The present invention also provides electrochemical cells containing at least one charge transport medium and at least two electrodes, in which the charge transport medium comprises or consists of an organic material having a specific conductivity of less than 10 -2 S/cm and a nonionic charge carrier mobility of greater than 10 -4 cm 2 /Vs, with the exception of electrochemical cells containing hexapentyloxytriphenylene or hexahexylthiotriphenylene as charge transport medium and containing transparent glass plates with an electroconductive coating as electrodes.

The novel use of organic materials having a specific conductivity of less than 10 -2 S/cm and a nonionic charge carrier mobility of greater than 10 -4 cm 2 /Vs as charge transport medium generally includes replacement of the electrolyte in electrochemical systems (for example electrical energy storage systems) and in particular as charge transport medium in electrochemical cells or electronic components.

Particular preference is given to their use in photoelectrochemical cells, in particular those containing a dye molecule embedded between two insulators. The insulators have energy states which enable the transport of charges, on the one hand of electrons in unoccupied states, and on the other hand of holes in occupied electronic energy states.

The electrodes or contact materials generally comprise metals, semimetals or semiconductors whose energy states and work functions enable charge transfer into the energy states provided for transport by the novel charge transport medium. The resultant contacts can have ohmic or rectifying current-voltage behavior, depending on their intended application. The contacting result in the formation of heterocontacts.

It is preferred according to the invention for one electrode (in general the semiconductor electrode) to have a nanocrystalline or fractal surface, as is preferably produced with the aid of sol/gel processes (for example "rough TiO 2 ").

In a preferred embodiment of the invention, the organic materials having a specific conductivity of less than 10 -2 S/cm and a non-ionic charge carrier mobility of greater than 10 -4 cm 2 /Vs are used as charge transport medium for charge carriers formed by exposing a dye (in sensitization-type solar cells, as described, for example, in DE-A-42 07 659) or an inorganic rectifying contact (in solid-state photovoltaics).

Accordingly, preference is given in accordance with the invention to electrochemical cells in which at least one of the electrodes comprises a rectifying semiconductor and/or is coated with a dye.

By contrast, indium tin oxide, with which glass plates are frequently coated to give electroconductivity, is a degenerate semiconductor which has no rectifying properties.

For the purposes of the present invention, electrochemical cells are, for example, batteries or accumulators, fuel cells, electrolyzers and galvanic cells.

For the purposes of the present invention, electronics components are in the broadest sense components in electronics, optoelectronics and/or molecular information storage and recording in which organic compounds having a specific conductivity of less than 10 -2 S/cm and a nonionic charge carrier mobility of greater than 10 -4 cm 2 /Vs are employed according to the invention as contact and window material for heterocontacts, metal replacement in Schottky contacts, p- and n-semiconducting material or as organic electrolyte replacement.

In particular, an organic material having liquid-crystalline properties is employed in accordance with the invention. Besides calamitic liquid-crystalline compounds, discotic liquid-crystalline compounds are particularly suitable.

Preference is given to organic materials having liquid-crystalline properties from the group consisting of unsubstituted and substituted triphenylenes, phthalocyanines, hexasubstituted benzenes, truxenes, hexa- and octasubstituted dibenzopyrenes and oligothiophenes.

Of these, particular preference is given to alkoxy- and thioalkyl-substituted triphenylenes and mixtures thereof, phthalocyanines, hexasubstituted benzenes, truxenes and hexa- and octasubstituted dibenzopyrenes, in particular alkoxy- and thioalkylsubstituted triphenylenes and mixtures thereof and hexa- and octasubstituted dibenzopyrenes.

A particularly preferred embodiment of the invention uses alkoxy-or thioalkyl-substituted triphenylenes or mixtures thereof. A very particularly preferred embodiment uses alkoxylated triphenylenes.

These organic materials are known per se (cf. German Patent Application P 43 39 711.5 and EP-A-0 527 376, and the references cited therein).

The novel use generally takes place in the form of thin coatings having a thickness of from 10 -9 to 10 -3 m. The coatings are preferably applied by vapor deposition, spin coating, knife coating, brushing or screen printing or are introduced, for example, into a cell arrangement by melting. In order to achieve the desired charge carrier mobilities, an alignment of the molecules is generally produced at above room temperature and is then frozen by slow cooling to the working temperature. Through suitable overlapping of energy states, the alignment on the molecules then provides conduction paths for charge carriers in which the latter then have an adequate mobility of, preferably, greater than 10 -3 cm 2 /Vs.

The charge carriers necessary for charge carrier transport are then themselves generally injected at least partly from the contact materials into the material and improve the conductivity of the organic material.

In a preferred embodiment of the novel electrochemical cell, the electrolyte comprises the organic material having a specific conductivity of less than 10 -2 S/cm and a nonionic charge carrier mobility of greater than 10 -4 cm 2 /Vs.

Further compounds or elements can be admixed with the charge transport medium, allowing further functionalization of the organic material itself. This includes, in particular, sensitization by light-absorbing substances, doping with electron acceptors or donors or the addition of reactive compounds whose properties sensitize the charge transport medium for substances which can be detected and measured in an intended sensor action.

The invention has numerous advantages. In the novel use, the production of charge carriers by charge carrier generation on absorption of light (photoionization) is not absolutely necessary. In addition, very simple contact production is possible. This can in principle be done at low temperatures. Passivation of the surfaces is simple, and the formation of defects during contacting is avoided. Furthermore, recombination losses with charged carriers in different bands generally do not occur. In addition, self-alignment with adaptation to non-planar surfaces can take place.

Furthermore, the use of organic compounds is associated with great variability in production of various band layers which allows targeted optimization of the overlapping of energy states to interfaces and contacts. This also allows adaptation of orbital layers of charge-carrier-generating and -transporting media.

In the novel use as charge transport medium for replacement of, for example, ionic electrolytes, the use of liquids which are difficult to handle industrially can be avoided. In addition, there is no need to search for ion-forming salts which have adequate solubility in the electrolyte, but which do not themselves participate in the charge transfer. Depending on the application, the use of redox systems which ensure charge transfer to the electrodes can also be avoided wholely or partly.

Furthermore, a change in the charge carrier mobility at defined temperatures, which occurs through phase conversions, can be utilized for switching processes.

Moreover, the novel organic materials have the advantage of extreme chemical and physical variability, which allows specific synthesis with respect to the properties required in each case. In addition, they have the advantage that, through targeted use of their various phase states (crystalline, liquid-crystalline, liquid and gaseous), in which the composition can remain unchanged, very simple construction of electrochemical cells and use of low production temperatures and inexpensive methods is possible. By contrast, only ion-conducting materials have hitherto been employed as charge transport medium (also referred to as electrolyte). Owing to their high charge carrier mobility, the novel organic materials can even dissipate charges in nonionic forms of contact materials or electrodes without themselves needing to have a high charge carrier concentration, either intrinsically, through doping or after photoexcitation. It is thus now possible to employ materials which have high resistances without externally injected charges.

The examples below illustrate the invention.